This invention relates generally to mechanical drive systems, and more particularly to a fail-safe spring drive system which is particularly suited for driving a syringe or other fluid infusion system over extended periods of time.
Known dispensers for infusing small doses of medical fluids over long periods of time are either bulky, and therefore not easily transportable, or powered by electrical motors. The motor-operated infusion systems require a source of power, such as electrical line power which confines the patient during the treatment, or batteries which are not reliable over long periods of time. Moreover, battery operated systems require the patient to maintain a relatively fresh supply of batteries since batteries have limited shelf lives.
It is a further problem with electrically operated dispenser systems that sophisticated and expensive insulation systems are required to prevent the introduction of even minute amounts of electrical energy into the vascular system of a patient. It is now known that even small amounts of electrical energy, illustratively on the order of microamperes, can adversely affect a patient's heart. Thus, in addition to affording only short periods of unattended operation, battery power infusion systems may be dangerous to the patient, particularly if the device is subject to wet conditions.
Some of the problems noted hereinabove associated with electrically operated dispenser systems are overcome by utilizing mechanical drive arrangements. Generally, the mechanical drive arrangements, in combination with a syringe, provide an infusion pump driven by a clock mechanism. The clock mechanisms are essentially of a conventional type wherein a plate-to-plate gearing system is provided a torque by a wound spring. The rate of rotation of the system, and consequently the rate of fluid infusion, is controlled by a conventional balance wheel and escapement system.
In addition to overcoming the disadvantages of electrical systems, mechanical drive systems provide all of the known economic advantages of time-released infusion systems. Thus, such mechanical systems permit continuous injection to the patient, thereby reducing labor requirements of hospital staff. Additionally, the automatic infusion systems reduce substantially the possibility of human failures, such as those which are produced when patient care personnel neglect or otherwise do not maintain prescribed injection schedules. Additionally, such mechanical systems provide the medical advantages of continuous injection, over cyclic injections, which conform more closely to the characteristics of fluid production systems within the patient.
It is a characteristic of conventional mechanical clockwork syringe drive systems that a high gear trained drive ratio, illustratively on the order of 1500:1 is required between the drive spring and escapement. Typically, the infusion apparatus is driven nearly directly by the spring. Four stages of spur gearing are generally required to achieve the required high gear ratios by plate-to-plate clockwork. When used in a clock, the four stages produce the hour, minute, and second hand drive, at the appropriate gear ratios. It is, however, a problem with such conventional clock technology that, when used as a clock, any failure of the gear drive mechanism is of relatively small consequence. At worst, the clock will show an incorrect time. Such is not the case, however, when a clock drive is utilized to pump a fluid intravenously into a patient. A failure of a clock mechanism under such circumstances may be catastrophic, and may result in a rapid release of the total spring energy into the infusion apparatus, illustratively a syringe, such that the patient receives a large, potentially lethal dosage of the infusion fluid. Additionally, conventional clock mechanisms are more prone to failure when used in an infusion system because substantially greater loads and stresses are required to drive a syringe than merely to rotate the hands of a clock.
In conventional plate-to-plate gear drive systems of the type which are generally provided in mechanical clock drives, a considerable shear stress is applied to the gear teeth since relatively few teeth along the perimeter of a gear are in engagement at any given time. Accordingly, each gear tooth is exposed to a substantial portion, and possibly the entire, drive load, resulting in a relatively high risk of failure. As noted hereinabove, the risk of failure is increased in infusion systems since the drive load is greater. Gear train failure in a plate-to-plate gear transmission system will almost always result in a catastrophic situation. For example, if the gear train fails at some point intermediate of the wound spring and the output shaft, the mechanism will stop and a patient may be denied the life-sustaining infusion. On the other hand, if the failure of the gear train occurs at a point intermediate of the output shaft and the escapement regulating system, then catastrophically rapid infusion may occur producing a potentially lethal overdose of the infusion fluid. Conventional clockwork systems, can therefore not be deemed to be sufficiently reliable in situations where human life may be at stake.
Failure of clockwork gear drive systems can occur from a variety of causes. For example, in a clockwork system, power is transmitted essentially from a plate gear to a pinion having substantially smaller diameter than the gear which drives it. Such pinions, however, are generally manufactured by known die casting or injection molded techniques. In such a manufacturing system, it may be possible that a defect exists in the mold or die which will result in a pinion tooth weakness which may be common to a batch of the devices. Of course, such weaknesses may also result from material or processing defects which are present in only one pinion. Such a weakness may result from an inclusion of foreign material during the casting or molding process. Such defects and weaknesses are generally not visible to the unaided eye, and can be detected with certainty only with expensive quality control systems such as x-ray imaging or ultrasonic systems.
It is, therefore, an object of this invention to provide a medical fluid infusion system which is inexpensive and simple to manufacture.
It is another object of this invention to provide a drive system for a syringe which is small, lightweight, portable, and easily attached to a patient.
It is also an object of this invention to provide a fluid dispensing system which is reliable over extended periods of operation.
It is a further object of this invention to provide a fail-safe arrangement for a fluid infusion system wherein uncontrolled operation of the fluid dispensing function is prevented.
It is also another object of this invention to provide a drive arrangement for a medical fluid dispensing system which does not require an external power source.
It is a yet further object of this invention to provide a medical fluid infusion system which does not require batteries for performing the fluid infusion function.
It is still a further object of this invention to provide a mechanical drive system which can provide a driving force at a controlled displacement rate.
It is additionally an object of this invention to provide a drive arrangement for a syringe utilizing only mechanically stored energy for performing the drive function.
It is yet another object of this invention to provide a high-reliability speed governor for a mechanical drive system.
It is yet a still further object of this invention to provide an operation indication for a vascular fluid infusion system.
It is additionally a further object of this invention to provide a substantially continuous power transmission arrangement whereby transmission torque is shared by a plurality of drive wheels.
Another object of this invention is to provide a drive system whereby the torque transmission capability between a drive shaft and an output shaft is substantially increased.
A further object of this invention is to provide a fail-safe system between an output shaft and a speed regulator thereof which prevents uncontrolled rotation of the regulator system in the event of tooth slippage in the regulator drive system.
A further object of this invention is to provide a fail-safe arrangement which prevents uncontrolled rotation of an escapement gear wheel.